We explain what the cell membrane is and some of its characteristics. In addition, its function and structure of this lipid layer.
What is the cell membrane?
A double layer of lipids that surrounds and delimits the cells , separating the interior from the outside and allowing the physical and chemical balance between the environment and the cytoplasm of the cell is called cell membrane, plasma membrane, plasmalema or cytoplasmic membrane . It is the outermost part of the cell.
This membrane is not visible to the optical microscope (yes to the electron microscope), since it has an average thickness of 7.3 nm3 and is located, in the plant and fungal cells, below the cell wall.
The primary characteristic of the cell membrane is its selective permeability, that is, its ability to allow or reject the entry of certain molecules into the cell, thus regulating the passage of water, nutrients or ionic salts, so that the cytoplasm is always in its optimal conditions of electrochemical potential (negatively charged), pH or concentration.
From this, two elementary processes of absorption (endocytosis) or expulsion (exocytosis) of substances from the cell can occur, which also allows the release of metabolic waste material.
This occurs through the formation of vesicles in the cell membrane that, depending on whether they enter or leave, allow dissolving the desired material in the cytoplasm or, conversely, in the environment.
The latter is vital in the case of certain unicellular cells and organisms that phagocytize (wrap in their membrane) the material for their nutrition or those agents that must be expelled from the organism (as in the case of lymphocytes or white blood cells).
Cell membrane function
The cell membrane fulfills the following functions:
- Delimitation . It mechanically defines and protects the cell, distinguishing the outside from the inside and one cell from another. In addition, it is the first defense barrier against other invading agents.
- Manages tion . Its selectivity allows it to give way to the desired substances in the cell and deny entry to the unwanted ones, serving as communication between the outside and the inside as well as the prosecutor of said transit.
- Preservation . Through the exchange of fluids and substances, the membrane allows the concentration of water and other solutes in the cytoplasm to be stable , maintaining its level pH and its constant electrochemical charge.
- Communication . The membrane can react to stimuli from outside, transmitting the information inside the cell and starting certain processes such as cell division, cell movement or segregation of biochemical substances.
Cell membrane structure
The cell membrane is composed of two layers of amphipathic lipids , whose hydrophilic polar heads (affinity for water) are oriented in and out of the cell, keeping their hydrophobic parts (which reject water) in contact, similar to a sandwich. Such lipids are primarily cholesterol, but also phosphoglycerides and sphingolipids.
It also has 20% of integral and peripheral proteins , which fulfill functions of connection, transport and catalysis. Thanks to them, cell recognition is also given, a form of biochemical communication.
And finally the cell membrane has carbohydrate components (sugars), whether they are polysaccharides or oligosaccharides, which are found on the outside of the membrane forming a glycocalix. These sugars represent only 8% dry of the total membrane weight, and serve as support material and as identifiers in intercellular communication.